There is a need on many differing types of plastic parts when only one area of the part has to have substantial wall thickness and the remainder of the part can be thinner. One example of this is the plastic lid on a trash dumpster. The main requirement of a lid on a dumpster is to keep rain and snow out and contain the trash. A lid can be made using very thin material molded with structural ribs that will satisfy both requirements. When such a lid is made of thin material, it typically fails by tearing around the pivot point where the lid is hinged to the container. The most typical method of hinging is by placing a steel rod through holes that are drilled in the ribs or knuckles that are formed in the plastic. The existing technology is that lids are now typically made of thicker material to toughen the hinge area, resulting in a higher cost.
A second example of this need for selectively increased wall thickness is a wall panel of a plastic shelter like a portable animal house for calves or any similar application. The upper portions of the wall are primarily for weather protection. Nearer to the bottom the walls are subject to considerable abuse and need to be stronger and thicker to facilitate fastening to the base, that is often times of another material like wood or solid plastic structural material. Additionally it may be necessary to have the wall material surround a majority of the base. Heretofore such a wall panel would be molded of thicker material in its entirety or constructed using a twinsheet thermoforming method, at additional cost.
There are numerous other applications also within this category. For example, such applications include liners in semi trailers where the bottom takes a majority of the abuse and the sidewalls of material handling containers to facilitate connecting the panels to the base.
Parts that are made using an injection, compression or casting molding process require an inner and outer mold that is completely filled with the plastic material. Any requisite additional thickness is accomplished by increasing the distance between the inner and outer mold halves and completely filling the space. Parts that are made by thermoforming or blow molding are limited by the starting stock or sheet thickness, typically with pressure or a vacuum on at least one side of the sheet or stock. The subject of this patent is to provide increased stock thickness when using the thermoforming or blow molding process.
Simple common thermoforming utilizes a single sheet of plastic that is heated then molded utilizing vacuum or pressure or both to draw the material into a one sided mold. Once formed, one side of the plastic is against the mold and the other side is open to the atmosphere for cooling. Typically this process is performed on a thermoforming machine of which there are many sizes and configurations.
There also exists a well-known thermoforming process called twin sheet molding. This processing method yields a finished part that is double walled, with inner and outer walls, with varying amounts of space between them. The two sheets of thermo plastic are married in the molding machine and typically fused together at particular areas in an invisible seam completely, or nearly so, around the perimeter and in various other areas advantageous to the application. Twinsheet thermoforming requires two sheets of plastic heated simultaneously and two molds mounted face to face in the molding machine. Typically twinsheeting is performed on a specially configured thermoforming machine of which there are many sizes. Any shape hollow part can be twinsheeted but the most typical application is large rectangular parts with only several inches or less separating the walls. Other processes are better suited for rounder more voluminous parts like tanks. The advantages of twinsheet thermoforming are numerous, the most important being the ability to make a hollow part that has considerable structural strength and the ability to have differing features on the top and bottom or opposite sides of the part.
Blow molding is a process that shares the same molding system as thermoforming, but the stock in the shape of a tube, called the parison, is extruded into a vertical standing open mold. When large flat parts are molded, a parison stretcher is employed. This mechanism essentially stretches the parison to extend beyond the edges of the mold. The extrusion process provides the heat for the blow molding process. When making a simple flat panel by blowmolding the mold is configured to provide two parts, face-to-face.
All three of the methods, simple and twinsheet thermoforming and blowmolding, require that the molded part be removed from the scrap material around its perimeter. This can be accomplished in many fashions but the preferable way when trimming parts is to rout them.
To enable the routing or other cutting of the parts from the scrap and to separate them, the molded parts must be shaped such that the router bit can separate the material but not cut through the primary part. Additionally, two primary parts as molded must have adequate distance between them to allow access for the trimming equipment. The preferable way to accomplish this is to have the material that creates a twinned band depart from the primary part at a 45 degree angle and then move at a steeper angle away until it becomes the primary part from the opposite cavity—thus providing the tool access space. The steeper angle minimizes the overall length of the tooling, saving cost of the raw material and the tooling.